- [feature] To complement [ticket:2547], types

- [feature] To complement [ticket:2547], types
can now provide "bind expressions" and
"column expressions" which allow compile-time
injection of SQL expressions into statements
on a per-column or per-bind level.   This is
to suit the use case of a type which needs
to augment bind- and result- behavior at the
SQL level, as opposed to in the Python level.
Allows for schemes like transparent encryption/
decryption, usage of Postgis functions, etc.
[ticket:1534]
- update postgis example fully.
- still need to repair the result map propagation
here to be transparent for cases like "labeled column".